In compressors of such type any refrigerant gas leaking around the peripheral surface of the reciprocating pistons flows into the rotor chamber and causes the pressure within the rotor chamber to increase. Conventionally the leaked refrigerant gas in the rotor chamber is returned to the suction side. Lubricating oil contained in a sump within the rotor chamber is stirred and atomized to lubricate all portions of the bearings and lubricating oil flows to the suction side together with the leaked refrigerant gas and is returned again into the rotor chamber. A disadvantage arises in that a large amount of lubricating oil is introduced into the refrigerating machine during the exhaust strokes and degrades the refrigerating capability while the temperature within the rotor chamber gradually increases and causes lowering of the lubricating capability.
Furthermore, as the pressure within the rotor chamber increases due to the leaked refrigerant gas, it is difficult to smoothly return the lubricating oil being introduced into the suction side again to the rotor chamber, and thereby a large amount of lubricating oil is contained at all times in the refrigerating cycle, whereby the refrigerating capability is considerably lowered.